1. Field of the Invention
The present invention relates to a vehicular airbag having a low mass and made substantially from thin plastic film which is designed to deploy in a collision involving the vehicle so that if it impacts the occupant of the vehicle wherever he/she is located, it will not cause significant injury to the occupant. In order to make a film airbag of sufficiently low mass so to not injure the occupant, it has been recognized that it must contain means to arrest the propagation of a tear so that a small hole or break in the film does not result in a catastrophic failure, i.e., cause the airbag to burst like a balloon or otherwise prevent the airbag from deploying properly. The particular method of arresting the propagation of a tear of this invention is to use a combination of an elastomeric film and reinforcement means which in certain embodiments may be the elastomeric material itself constructed in a variable thickness pattern, i.e., have thinner and thicker sections, or in a manner so that it has strategically placed thicker sections, i.e., relative to remaining portions of the material, in view of stress considerations during deployment.
2. Description of the Prior Art
A conventional driver side airbag (also referred to herein as a driver airbag) is made from pieces of either Nylon or polyester fabric which are joined together, e.g., by sewing. The airbag is usually coated on the inside with neoprene or silicone for the purposes of (i) capturing hot particles emitted by the inflator in order to prevent holes from being burned in the fabric, and (ii) sealing the airbag to minimize the leakage of an inflating gas through the fabric. These airbags are conventionally made by first cutting two approximately circular sections of a material having a coating on only one side and which will form a front panel and a back panel, and sewing them together with the coated side facing out. The back panel is provided with a hole for attachment to an inflator. Fabric straps, called tethers, are then sewn to the front panel. Afterwards, the airbag is turned inside out by pulling the fabric assembly through the inflator attachment hole placing the coated side on the inside. Assembly is completed by sewing the tethers to the back panel adjacent the inflator attachment hole.
If a conventional driver airbag is inflated without the use of tethers, the airbag will usually take an approximately spherical shape. Such an inflated airbag would protrude significantly into the passenger compartment from the steering wheel and, in most cases, impact and injure the driver. To prevent this possible injury, the tethers are attached to the front and rear panels of the airbag to restrict the displacement of the front panel relative to the back panel. The result of the addition of such tethers is an airbag which has the shape of a flat ellipsoid with a ratio of the thickness of the airbag to its diameter of approximately 0.6. In the conventional airbag, the tethers are needed since the threads which make up the airbag fabric are capable of moving slightly relative to each other. The airbag is elastic for stresses which are not aligned with the warp or woof of the fabric. As a result, the fabric would distort to form an approximate sphere in the absence of such tethers.
Moreover, the above-mentioned method of manufacturing an airbag involves a great deal of sewing and thus is highly labor intensive and, as a result, a large percentage of all driver airbags are presently manufactured in low labor cost countries such as Mexico.
Many people are now being injured and some killed by interaction with the deploying airbag (See, e.g., "Warning: Too Much Safety May Be Hazardous", New York Times, Sunday, Dec. 10, 1995, Section F, Page 8). One of the key advantages of the film airbag described in the current assignee's above-referenced patent applications is that, because of its much lower mass than conventional Nylon or polyester fabric airbags, the injury caused by this interaction with the deploying airbag is substantially reduced. In accordance with the teachings of those patent applications mentioned above, the driver airbag system can be designed to permit significant interaction with the driver. In other words, the film airbag can be safely designed to intrude substantially further into the passenger compartment without fear of injuring the driver. Nevertheless, in some cases, as disclosed in patent application Ser. No. 08/539,676, it may be desirable to combine the properties of a film airbag, which automatically attains the conventional driver airbag shape, with a fabric airbag. In such cases, interaction with the driver needs to be minimized.
Airbag systems today are designed so that ideally the airbag is fully inflated before the occupant moves into the space which is occupied by the airbag. However, most occupants are not positioned at the ideal location assumed by the airbag system designer, and also may not have the dimensions, e.g., size and weight, in the range considered for optimum airbag deployment by the airbag system designer. Many occupants sit very close to the airbags, or at least closer than expected by the airbag system designer, and as mentioned above, are injured by the airbag deployment. On the other hand, others sit far from the airbag, or at least farther away from the airbag than expected, and therefore must travel some distance, achieving a significant relative velocity, before receiving the benefit of the airbag. See for example "How People Sit in Cars: Implications For Driver and Passenger Safety in Frontal Collisions--The Case for Smart Restraints.", Cullen, E., et al 40.sup.th Annual Proceedings, Association For the Advancement of Automotive Medicine, pp. 77-91.
With conventionally mounted airbags such as those mounted in the steering wheel or instrument panel, severe out-of-position occupant situations, where the occupant is resting against the airbag when deployment begins, can probably only be handled using an occupant position sensor, such as disclosed in the current assignee's copending patent application Ser. No. 08/505,036 (corresponding to published WO 94/22693), which is incorporated herein by reference, which prevents an airbag from deploying if an occupant is more likely to be seriously injured by the airbag deployment than from the accident itself. In many less severe accidents, the occupant will still interact with the deploying airbag and sustain injuries ranging from the mild to the severe. In addition, as mentioned above, some occupants sit very far from the steering wheel or instrument panel and, with conventional airbags, a significant distance remains between the occupant and the inflated airbag. Such occupants can attain a significant kinetic energy relative to the airbag before impacting it, which must be absorbed by the airbag. This effect serves to both increase the design strength requirements of the airbag and increase the injury induced in the occupant by the airbag. For these reasons, it would be desirable to have an airbag system that adjusts to the location of the occupant and which is designed so that the impact of the airbag causes little or no injury to the occupant.
It is conventional in the art that airbags contain orifices or vent holes for exhausting or venting the gas generated by the inflation means. Thus, typically within one second after the bag is inflated (and has provided its impact absorbing function), the gas has been completely exhausted from the bag through the vent holes. This imposes several limitations on the restraint system which encompasses the airbag system. Take for example the case where an occupant is wearing a seatbelt and has a marginal accident, such as hitting a small tree, which is sufficient to deploy the airbag, but where it is not really needed since the driver is being restrained by his seatbelt. If the driver has lost control of the car and is traveling at 30 MPH, for example, and has a secondary impact one second or about 50 feet later, this time with a large tree, the airbag will have become deflated and thus is not available to protect the occupant in this secondary life threatening impact.
In other situations, the occupant might be involved in an accident which exceeds the design capability of the restraint system. These systems are typically designed to protect an average-size male occupant in a 30 MPH barrier impact. At higher velocities, the maximum chest deceleration experienced by the occupant can exceed 60 G's and become life threatening. This is particularly a problem in smaller vehicles, where airbag systems typically only marginally meet the 60 G maximum requirement, or with larger or more frail occupants.
There are many cases, particularly in marginal crashes, where existing crash sensors will cause the airbag to deploy late in the crash. This can also result in an "out-of-position occupant" for deployment of the airbag which can cause injuries and possibly death to the occupant. Other cases of out-of-position occupants are standing children or the forward motion of occupants during panic braking prior to impact especially when they are not wearing seatbelts. The deploying airbag in these situations can cause injury or death to the out-of-position occupant. Approximately fifty people have now been killed and countless more seriously injured by the deployment of the airbag due to being out-of-position.
It is recognized in the art that the airbag must be available to protect an occupant for at least the first 100-200 milliseconds of the crash. Since the airbag contains large vents, the inflator must continue to supply gas to the airbag to replace the gas flowing out of these vents. As a result, inflators are usually designed to produce about twice as much gas than is needed to fill the airbag. This, of course, increases the cost of the airbag system as well as its size, weight and total amount of contaminants resulting from the gases which are exhausted into the automobile environment.
This problem is compounded when the airbag becomes larger, which is now possible using the film materials of this invention, so as to impact with the occupant wherever he/she is sitting, without causing significant injury, as in the preferred implementation of the design of this invention. This then requires an even larger inflator which, in many cases, cannot be accommodated in conjunction with the steering wheel, if conventional inflator technology is utilized.
Furthermore, there is a great deal of concern today for the safety of a child in a rear facing child seat when it is used in the front passenger seat of a passenger airbag equipped vehicle. Currently used passenger side airbags have sufficient force to cause significant injury to a child sitting in such a seat and parents are warned not to use child seats in the front seat of a vehicle having a passenger side airbag. Additionally, several automobile companies are now experimenting with rear seat airbags in which case, the child seat problem would be compounded.
Airbags made of plastic film are disclosed in the copending patent applications referenced above. Many films have the property that they are quite inelastic under typical stresses associated with an airbag deployment. If an airbag is made from a pair of joined flat circular sections of such films and inflated, instead of forming a spherical shape, it automatically forms the flat ellipsoidal shape required for driver airbags as disclosed in copending U.S. patent application Ser. No. 08/539,676. This unexpected result vastly simplifies the manufacturing process for driver airbags since tethers are not required, i.e., the film airbag is made from two pieces of film connected only at their peripheral edges. Furthermore, since the airbag can be made by heat sealing two flat circular sections together at their peripheral edges without the need for tethers, the entire airbag can be made without sewing, reducing labor and production costs. In fact, the removal of the requirement for tethers permits the airbag to be made by a blow molding or similar process. Indeed, this greatly reduces the cost of manufacturing driver airbags. Thus, the use of film for making an airbag has many advantages which are not obvious.
Films having this inelastic quality, that is films with a high modulus of elasticity and low elongation at failure, tend to propagate tears easily and thus when used alone are not suitable for airbags. This problem can be solved through the addition of reinforcement in conjunction with the inelastic films such as a net material as described in the above-referenced patent applications. Other more elastic films such as those made from the thermoplastic elastomers, on the other hand, have a low modulus of elasticity and large elongation at failure, sometimes 100%, 200% or even 400%, and naturally resist the propagation of tears. Such films, on the other hand, do not form the flat ellipsoidal shape desired for steering wheel mounted driver side airbags. As discussed in greater detail below, the combination of the two types of film through attachment using lamination, successive casting or coating, or through the use of adhesives applied in a pattern can produce a material having both the self shaping and the resistance to tear propagation properties.
In addition to the above-referenced patent applications, film material for use in making airbags is described in U.S. Pat. No. 4,963,412 to Kokeguchi, which is incorporated herein by reference. The film airbag material described in the Kokeguchi patent is considerably different in concept from that disclosed in the above-referenced patent applications or the instant invention. The prime feature of the Kokeguchi patent is that the edge tear resistance, or notch tear resistance, of the airbag film material can be increased through the use of holes in the plastic films, i.e., the film is perforated. Adding holes, however, reduces the tensile strength of the material by a factor of two or more due to the stress concentration effects of the hole. It also reduces the amount of available material to resist the stress. As such, it is noteworthy that the Kokeguchi steering wheel mounted airbag is only slightly thinner than the conventional driver side fabric airbag (320 micrometers vs. the conventional 400 micrometers) and is likely to be as heavy or perhaps heavier than the conventional airbag. Also, Kokeguchi does not disclose any particular shapes of film airbags or even the airbag itself for that matter. Since his airbag has no significant weight advantage over conventional airbags, there is no teaching in Kokeguchi of perhaps the most important advantage of film airbags of the present invention, that is, in reducing injuries to occupants who interact with a deploying airbag.
As will be discussed in detail below, the airbags constructed in accordance with the present teachings attain particular shapes based on the use of the inelastic properties of particular film materials and reduce tear propagation through a variety of novel methods including the use of elastic films. It is also noteworthy that Kokeguchi discloses that vacuum methods can be used to form the airbag into the desired shape and thus fails to realize that the properties of inelastic film results in the airbag automatically forming the correct shape upon deployment. Also noteworthy is that Kokeguchi states that polymeric films do not have sufficient edge tear resistance and thus fails to realize that films can be so formulated to have this property, particularly those made from the thermoplastic elastomers. These limitations of the Kokeguchi patent results in a very thick airbag which although comprised of film layers no longer qualifies as a true film airbag as defined herein. A "film airbag" for the purposes herein is one wherein the film thickness is generally less than about 250 micrometers, and preferably even below about 100 micrometers, for use as a driver protection airbag. As the size of the airbag increases, the thickness must also increase in order to maintain an acceptable stress within the film. A film airbag so defined may also contain one or more sections which are thicker than about 250 micrometers and which are used primarily to reinforce the thinner film portion(s) of the airbag. A film airbag as defined herein may also include a layer or layers of inelastic material and a layer or layers of elastic material (i.e., thermoplastic elastomers).
The neoprene or silicone coating on conventional driver airbags, as mentioned above, serves to trap hot particles which are emitted from some inflators, such as a conventional sodium azide inflator. A film airbag may be vulnerable to such particles, depending on its design, and as a result cleaner inflators that emit fewer particles are preferred over sodium azide inflators. It is noteworthy, however, that even if a hole is burned through the film by a hot particle, the use of an thermoplastic elastomer in the film material prevents this hole from propagating and causing the airbag to fail. Also, new inflators using the pyrotechnic, hybrid or stored gas technologies, are now available which do not produce hot particles and produce gases which are substantially cooler than gases produced by sodium azide inflators. Also, not all sodium azide inflators produce significant quantities of hot particles.
One interesting point which also is not widely appreciated by those skilled in the art heretofore, is that the gas temperature from the inflator is only an issue in the choice of airbag materials during the initial stages of the inflation. The total thermal energy of the gas in an airbag is, to a first order approximation, independent of the gas temperature which can be shown by application of the ideal gas laws. When the gas initially impinges on the airbag material during the early stages of the inflation process, the temperature is important and, if it is high, care must be taken to protect the material from the gas. Also, the temperature of the gas in the airbag is important if the vent holes are located where the out-flowing gas can impinge on an occupant. The average temperature of the airbag itself, however, will not be affected significantly by the temperature of the gas in the airbag.
In certain conventional airbag deployments, the propellant which is used to inflate the airbag also is used to force open a hole in the vehicle trim, called the deployment door, permitting the airbag to deploy. Since the mass of a film airbag is substantially less than the mass of a conventional fabric airbag, much less energy is required to deploy the airbag in time. However, substantial pressure is still required to open the deployment door. Also, if the pressure now used to open the deployment door is used with film airbags, the airbag velocity once the door has been opened may be substantially higher than conventional airbags. This rapid deployment can put excessive stresses on the film airbag and increases the chance that the occupant will be injured thereby. For most implementations of the film airbag, an alternate less energetic method of opening the deployment door may be required.
One such system is disclosed in Barnes et al. (U.S. Pat. No. 5,390,950) entitled "Method and arrangement for forming an air bag deployment opening in an auto interior trim piece". This patent describes a method ". . . of forming an air bag deployment opening in an interior trim piece having a vinyl skin overlying a rigid substrate so as to be invisible prior to operation of the air bag system comprising an energy generating linear cutting element arranged in a door pattern beneath the skin acting to degrade or cut the skin when activated."
The goal of the Barnes et al. patent is to create an invisible seam when the deployment door is located in a visible interior trim panel. This permits greater freedom for the vehicle interior designer to create the particular aesthetic effect that he or she desires. The invisible seam of the Barnes et al. patent is thus created for aesthetic purposes with no thought toward any advantages it might have to reduce occupant injury or advantages for use with a film airbag, or to reduce injuries at all for that matter. One unexpected result of applying the teachings of this patent is that the pressure required to open the deployment door, resulting from the force of the inflating airbag, is substantially reduced. When used in conjunction with a film airbag, this result is important since the inflator can be designed to provide only sufficient energy to deploy and inflate the very light film airbag thereby significantly reducing the size of the inflator. The additional energy required to open a conventional deployment door, above that required to open a deployment door constructed in accordance with the teachings of the Barnes et al. patent, is not required to be generated by the inflator. Furthermore, since a film airbag is more vulnerable to being injured by ragged edges on the deployment door than a conventional fabric airbag, the device of the Barnes et al. patent can be used to pyrotechnically cut open the deployment door permitting it to be easily displaced from the path of the deploying airbag, minimizing the force of the airbag against the door and thus minimizing the risk of damage to the film airbag from the deployment door. Since Barnes et al. did not contemplate a film airbag, advantages of its use with the pyrotechnically opening deployment door could not have been foreseen. Although the Barnes et al. patent discloses one deployment door opening method which is suitable for use with an airbag made from plastic film as disclosed herein, that is one which requires substantially less force or pressure to open than conventional deployment doors, other methods can be used in accordance with the invention without deviating from the scope and spirit thereof.
A finite element analysis of conventional driver side airbags (made of fabric) shows that the distribution of stresses is highly unequal. Substantial improvements in conventional airbag designs can be made by redesigning the fabric panels so that the stresses are more equalized. Today, conventional airbags are designed based on the strength required to support the maximum stress regardless of where that stress occurs. The entire airbag must then be made of the same thickness material as that chosen to withstand maximum stress condition. Naturally, this is wasteful of material and attempts have been made to redesign the airbag to optimize its design in order to more closely equalize the stress distribution and permit a reduction in fabric strength and thus thickness and weight. However, this optimization process when used with conventional fabric airbags can lead to more complicated assembly and sewing operations and more expensive woven materials and thus higher overall manufacturing costs. An example of such an airbag is that marketed by Precision Fabrics of Greensboro, N.C. Thus, there is a tradeoff between manufacturing cost and airbag optimization.
As discussed in the above-referenced patent applications as well as below, with a film airbag manufactured using blow molding techniques, for example, greater freedom is permitted to optimize the airbag vis-a-vis equalization of the stress. First, other than tooling cost, the manufacturing cost of an optimized airbag is no greater than for a non-optimized airbag. Furthermore, the thickness of the film can be varied from one part of the airbag to another to permit the airbag to be thicker where the stresses are greater and thinner where the stresses are less. A further advantage of blow molding is that the film can be made of a single constituent material. When the airbag is fabricated from sheet material, the outside layer of the material needs to be heat sealable, such as is the case with polyethylene or other polyolefin, or else a special adhesive layer is required where the sealing occurs.
As discussed in greater detail below in connection with the description of the invention, when the film for the airbag is manufactured by casting or coating methods, techniques familiar to those skilled in the art of plastics manufacturing are also available to produce a film where the thickness varies from one part to another in a predetermined pattern. This permits a film to be made which incorporates thicker sections in the form of a lattice, for example, which are joined together with thin film. Thus, the film can be designed so that reinforcing ribs, for example, are placed at the optimum locations determined by mathematical stress analysis.
One example of an inflatable film product which partially illustrates the self shaping technology of this invention is the common balloon made from metalized "Myla".TM. plastic film found in many stores. Frequently these balloons are filled with helium. They are made by heat sealing two flat pieces of film together as described in U.S. Pat. Nos. 5,188,558 (Barton), 5,248,275 (McGrath), 5,279,873 (Oike), and 5,295,892 (Felton). Surprisingly, the shape of these balloons, which is circular in one plane and elliptical in the other two planes, is very nearly the shape which is desired for a driver side airbag. This shape is created when the pressure within the balloon is sufficiently low such that the stresses induced into the film are much smaller than the stresses needed to significantly stretch the film. The film used is relatively rigid and has difficulty adjusting to form a spherical shape. In contrast, the same airbag made from woven material more easily assumes an approximate spherical shape requiring the use of tethers to create the shape which comes naturally with the Mylar balloons.
One problem with film balloons is that when a hole is punctured in the balloon it fails catastrophically. One solution to this problem is to use the combination of a film and net as described in the current assignee's above-referenced patent applications. Such materials have been perfected for use as sail material for lightweight high performance sails for sailboats. One example is marketed under the trade name Bainbridge Sailcloth SL Series.TM., and in particular SL 500-P.TM., 1.5 mill. This material is a laminate of a film and a net. Such materials are frequently designed to permit heat sealing thereby eliminating threads and the stress concentrations associated therewith. Heat sealing also simplifies the manufacturing process for making sails. Another preferable solution is to make the airbags from a film material which naturally resists tears, that is, one which is chemically formulated to arrest a tear which begins from a hole, for example. Examples of films which exhibit this property are those from the thermoplastic elastomer (TPE) families such as polyurethane, Ecdel elastomer from Eastmen, polyester elastomers such as HYTREL.TM. and some metallocene catalyzed polyolefins. For the purposes herein, a thermoplastic elastomer will include all plastic films which have a relatively low modulus of elasticity and high elongation at failure, including but not limited to those listed above.
Applications for the self shaping airbag described herein include all airbags within the vehicle which would otherwise required tethers or complicated manufacturing from several separate panels. Most of these applications are more difficult to solve or unsolvable using conventional sewing technology. The invention described herein solves some of the above problems by using the inelastic properties of film, and others by using the elastic properties of thermoplastic elastomers plus innovative designs based on analysis including mathematical modeling plus experimentation. In this manner, the problems discussed above, as well as many others, are alleviated or solved by the airbags described in the paragraphs below. Films for airbags which exhibit both the self shaping property and also formulated to resist the propagation of a tear are made by combining a layer of high modulus material with a layer of a thermoplastic elastomer. Then if a tear begins in the combined film it will be prevented from propagating by the elastomer yet the airbag will take the proper shape due to the self shaping effect of the high modulus film.
Other relevant prior art includes the following, with a brief explanation of the pertinence of the reference to the present invention:
U.S. Pat. No. 3,511,519 (Martin) describes a large fabric airbag which is shown impacting the occupant. It does not discuss the problem of injury to the occupants due to the impact of the airbag.
U.S. Pat. No. 4,262,931 (Strasser) illustrates two airbags joined together to cover right and center seating positions.
U.S. Pat. No. 3,451,693 (Carey) describes plastic airbags but not plastic film airbags. The distinguishable properties of film are numerically described in the above referenced copending patent application (ATI-146) and basically are its thinness and lower weight.
Japanese Patent No. 89-090412/12 describes fabricated cloths are laminated in layers at different angles to each other's warp axis to be integrated with each other. Strength and isotropy are improved. The cloth is stated as being useful for automotive air bags for protecting the passenger's body.
U.S. Pat. No. 5,322,326 (Ohm) describes a small limited protection airbag manufactured in Korea. Although not disclosed in the patent, it appears to use a plastic film airbag material made from polyurethane. It is a small airbag and does not meet the United States standards for occupant protection (FMVSS-208). The film has a uniform thickness and if scaled to the size necessary for meeting U.S. Standards it would likely become of comparable thickness and weight as the current fabric airbags.